Electron microprobe investigations of metamorphic reactions and mineral growth histories, Kwoiek area, British Columbia

Metamorfismo ercinico di bassa-pressione: evoluzione tettonico-metamorfica del complesso di mandatoriccio(massiccio della Sila - Calabria)

The inner zone of the Sardinia Variscan segment consists of two metamorphic complexes: I) A polymetamorphic Migmatite Complex, with migmatites showing polyphase anatectic processes, in the presence of kyanite or sillimanite. The Migmatite complex preserved decametric lenses of eclogite relicts (eclogites A) affected by high T, high- to intermediate P recrystallization under granulite facies conditions The decompressional garnet + Ca-clinopyroxene + amphibole ± orthopyroxene-bearing assemblages developed in granoblastic textures generally in no stress conditions. In most cases, only symplectite textures provide evidence for the eclogitic event. II) A medium grade, mostly metapelitic complex consisting of Grt, Ky, Stau-bearing micaschists and paragneisses includes quartzites and garnet-bearing amphibolite boudins with N-MORB chemical affinity. Relicts of eclogite assemblages were locally found in the metabasite (eclogites B). In eclogites A, the geothermobarometric parameters yield temperatures in the range 690°-760°C for minimum pressure A1.3 GPa. Pyroxene compositions accord with temperatures in excess of 700°C. In eclogites B, the thermometric calibrations provide temperatures in the range 610°-700°C for pressures 1.3-1.5 GPa, based on the jadeite content. The temperatures are consistent with the biotite+muscovite+garnet+kyanite+staurolite assemblage in the host paragneisses, and with lack of anatectic processes. The age of 457±2 Ma, obtained by U/Pb dating on one sample of Type A eclogite is interpreted as a minimum estimate for the magmatism of the eclogite protolith. A second zircon population defined an age of 403±4 Ma interpreted as dating the zircon crystallization during the high-grade event. The relationships between Types A and B eclogites, and their bearing on the regional framework (Sardinia, Ligurian Alps) are discussed.

The Variscan basement of Northern Apennines (Northern Italy) is a polymetamorphic portion of continental crust. This thesis investigated the metamorphic history of this basement occurring in the Cerreto Pass, in the Pontremoli well, and in the Pisani Mountains. The study comprised fieldwork, petrography and microstructural analysis, determination of the bulk rock and mineral composition, thermodynamic modelling, conventional geothermobarometry, monazite chemical dating and Ar/Ar dating of muscovite. The reconstructed metamorphic evolution of the selected samples allowed to define a long-lasting metamorphic history straddling the Variscan and Alpine orogenesis. Some general petrological issues generally found in low- to medium-grade metapelites were also tackled: (i) With middle-grade micaschist it is possible to reconstruct a complete P-T-D path by combining microstructural analysis and thermodynamic modelling. Prekinematic white mica may preserve Mg-rich cores related to the pre-peak stage. Mn-poor garnet rim records the peak metamorphism. Na-rich mylonitic white mica, the XFe of chlorite and the late paragenesis may constrain the retrograde stage. (ii) Metapelites may contain coronitic microstructures of apatite + Th-silicate, allanite and epidote around unstable monazite grains. Chemistry and microstructure of Th-rich monazite relics surrounded by this coronitic microstructure may suggest that monazite mineral was inherited and underwent partial dissolution and fluid-aided replacement by REE-accessory minerals at 500-600°C and 5-7 kbar. (iii) Fish-shaped white mica is not always a (prekinematic) mica-fish. Observed at high-magnification BSE images it may consist of several white mica formed during a mylonitic stage. Hence, the asymmetric foliation boudin is a suitable microstructure to obtain geochronological information about the shearing stage. (iv) Thermodynamic modelling of a hematite-rich metasedimentary rock fails to reproduce the observed mineral compositions when the bulk Fe2O3 is neglected or determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P-XFe2O3 diagrams.

مجموعه گرانیتوئیدی کلاه قاضی در جنوب اصفهان و در پهنه ساختاری سنندج‌- سیرجان واقع شده است. بر اساس شواهد زمین‌شناسی سن احتمالی این مجموعه ژوراسیک بالایی بوده و شامل گروه های سنگی گرانودیوریت، گرانیت و آلکالی فلدسپار گرانیت است. کانی های اصلی تشکیل‌دهنده این مجموعه شامل کوارتز، پلاژیوکلاز و فلدسپار آلکالن بوده و بیوتیت تنها کانی فرومنیزین این مجموعه نفوذی است. حضور کانی های آندالوزیت، سیلیمانیت و گارنت در این مجموعه گرانیتوئیدی نشان‌دهنده منشأ رسوبی توده های مورد بررسی است. بر اساس مطالعات صحرایی، پتروگرافی و ژئوشیمیایی، سنگ‌های مجموعه نفوذی مورد بررسی در گروه گرانیتوئیدهای نوع S قرار می گیرند و دارای ماهیت کالک‌آلکالن پتاسیم بالا و از نوع پرآلومین هستند. از لحاظ ژئوشیمیایی طیف تغییرات SiO2 در نمونه های آنالیزشده از 6/64 تا 4/74 متغیر است. الگوی تغییرات عناصر کمیاب بهنجار شده نسبت به گوشته اولیه و کندریت بیانگر غنی‌شدگی این سنگ‌ها از LILE و LREE و تهی شدگی از HFSE و HREE است. نسبت 85/0-28/0 Eu/Eu*= در نمونه های کلاه قاضی نشان‌دهنده حضور اندکی پلاژیوکلاز در باقی‌مانده ذوب در منبع است. بر اساس شواهد زمین‌شناسی، کانی شناسی و ژئوشیمیایی مجموعه نفوذی کلاه قاضی در ژوراسیک بالایی و در طی نفوذ مذاب‌های گوشته ای به داخل پوسته و ذوب‌بخشی سنگ‌های پوسته ای تشکیل شده است.

The PALMEIROPOLIS Cu-Zn (Pb) volcanogenic massive sulfide deposit, Brazil, consists of three ore bodies enclosed by hydrothermal alteration zones. The ore bodies and the alteration zones were metamorphosed under amphibolite fades conditions. The Palmeirdpolis alteration zones are characterized by a great diversity of bulk rock composition that originated a wide variety of low variance mineral assemblages. These assemblages are composed of orthoamphiboles (anthophyllite and gedrite), hornblende, biotite, garnet, staurolite, sillimanite, gahnite and, rarer, cordierite. Based on analyses of mineral chemistry aad mineral assemblages, temperatures are estimated to have been 550 - 625°C and pressures 2 - 5.5 kbar. The temperature of metamorphism that prevailed at the Palmeirdpolis deposit is comparable to other amphibolite fades massive sulfide deposits, such as Geco and Linda, Canada; Falun, Sweden; and Bleikvassli, Norway. The mineralogy of the alteration zones is similar in all these deposits even though they were metamorphosed at different pressure conditions, reflected by the crystallization of one of Al2 SiO5 phase (andalusite, sillimanite or kyanite).

In the Cerreto area metamorphic rocks belonging to the basement of the Northern Apennine are exposed in association with Triassic evaporites and quartzites. The Cerreto basement rocks consist of lenses and plurimetric bodies of amphibolites included in a sequence of metapsammites (biotite and/or muscovite schists). Both metasediments and amphibolites underwent a retrograde evolution starting from an initial medium-pressure amphibolite stage (M1: P ~ 8 kbar, T ~ 650°C) down to greenschist condition. The amphibolites show a well preserved earlier foliation associated with development of green amphibole (aluminotschermakite) + plagioclase + quartz + ilmenite assemblage followed by partial retrograde recrystallization in epidote-amphibolite facies under static conditions and along shear zones (M2: ~ P 4-5 kbar, T ~ 530°C); pre-kinematic garnet porphyroblasts rimmed by amphibole + plagioclase coronas are widespread. Metasediments show a pervasive mylonitic deformation associated with amphibolite to greenschist facies assemblages and rare relics of an earlier fabric, testified by inclusions of quartz + biotite in plagioclase and garnet porphyroclasts. The latest stages of deformation are accompanied in both metasediments and amphibolites by retrograde development of greenschist-facies assemblages in millimeter-scale shear zones crosscutting the former structures. Major and trace element compositions of the amphibolites suggest that most protoliths are cogenetic and could be related through fractional crystallization of plagioclase + olivine and/or pyroxene from variably evolved tholeiitic liquids. Geochemical data also indicate that most of the amphibolites have protolith features resembling Enriched-type MORB. 40Ar/39Ar dating performed on two hornblende samples (312 and 328 Ma) indicate a Variscan age for the amphibolite-facies metamorphism implying that the Cerreto metamorphic rocks belong to a unit deformed and metamorphosed at deep structural levels during the main deformational event (i.e. collisional stage) of the Southern Europe Variscan belt. The present tectonic setting of the Cerreto rocks, i.e. rootless slices within cover unit, could be related to the Apenninic thrusting. The Apenninic tectonic history likely occurred at very high structural levels without development of pervasive syn-metamorphic structures.

It is the purpose of the present paper to criticize the hypothesis of stress-minerals proposed by Alfred Harker, to make it clear that it is unneccessary to use such a concept, and to attempt the explanation of metamorphic rock characteristics by regarding temperature, pressuer (hydrostatic), and concentration of components as the only essential factors controlling metamorphism., Petrological and physico-chemical considerations do not show the necessity of Harker's hypothesis of stress-minerals, in the present writers' view., All the minerals which Harker ranked in the category of stress-minerals, seem to be capable of formed under some conditions without stress., They may be divided into two groups, according to the condition of generation., The first group including kyanite, staurolite, almandine, and chloritoids, may be formed only under very high pressures., They all have very high density., They occur sometimes in ignenous rocks, pegmatitic veins, hornfelses, etc., which do not show any special evidence of stress., The second group, containing chlorite, muscovite, epidote, etc., may be formed only at low temperatures., This is shown by the fact that they are hydrothermal or deuteric minerals, the temperature of whose formation is much lower than that of ordinary pyrogenetic or thermal-metamorphic minerals., Harker's stress-minerals seem to be such as are formed only under conditions of lower temperatures and perhaps higher pressures than in ordinary hornfelses., Each of various kinds of metamorphism, such as thermal, regional, etc., takes place under a certain definite condition of temperature and pressure., The kinds of metamorphic minerals developed are determined by the prevailing temperature and pressure as well as by the chemical composition of the metamorphosed rocks., Therefore, each kind of metamorphism is distinguished from the others by a characteristic set of metamorphic minerals.,

In situ U-Pb geochronology was carried out on amphibolites and siliciclastic metasediments of the Kinzigite Formation exposed in the northernmost sector of the Ivrea-Verbano Zone (Finero area). The aim is to shed light on the tectono-metamorphic evolution of this intermediate-lower crustal section and its bearing with the evolution of the southern and better known sectors of the IVZ. Based on field observation and petrography a metamorphic gradient gently increasing from amphibolite to upper amphibolite facies (from SE to NW) characterizes the whole metamorphic sequence. Metapelites consist mainly of biotite, quartz, plagioclase, garnet, and sillimanite; muscovite progressively disappears as K-feldspar appears and becomes abundant. Amphibolites are made of green-brown hornblende and plagioclase and may contain clinopyroxene defining thin layers together with plagioclase and titanite. Both metapelites and amphibolites show mylonitic deformation which is more intense towards NW, i.e. towards the lower structural levels. The mylonitic deformation strongly affected the lower crustal metabasic rocks of External Gabbro unit (Finero Mafic Complex). Zircon, monazite and titanite U-Pb geochronology was carried out with laser ablation (LA)-ICP-MS on amphibolites, migmatitic paragneiss and mafic granulites occurring as slivers of the Kinzigite Formation in the External Gabbro unit of the Finero Mafic Complex. The multi-chronological approach allowed recognizing three discrete tectono-metamorphic events, at Permian, Triassic and Jurassic. Zircon and monazite yielded Permian ages suggesting (re)crystallization during an high temperature event characterized by both metamorphism and magmatism. Titanite dating provided Triassic and Jurassic ages that were interpreted as U-Pb resetting ages. A Triassic perturbation of the U-Pb system was also recorded by zircon and monazite as rare domains. The tectono-metamorphic reconstruction of the evolution of the northernmost IVZ, as revealed by the new geochronological data, is only partially in agreement with the temperature-time evolutions depicted for the southern sectors of the IVZ. Permian ages indicating magmatism and high temperature metamorphism are common throughout the IVZ, as well as the Jurassic ages related to local thermal pulses and tectonic activity. Conversely, the occurrence of well-constrained Triassic ages is in fact peculiar of the Finero area. Two possible explanations may account for this Triassic event: Triassic ages are possibly related to the thermal effect and fluid circulation during the emplacement of the External Gabbro unit; or alternatively, they are the response to the ductile deformation largely recognized in the whole area. This study is a further evidence of the necessity of approaching crystalline basement with multiple geochronometers in order to unravel the complete tectono-metamorphic evolution.

Introduction The Avan Cu-Fe skarn is located at the southern margin of Qaradagh batholith, about 60 km north of Tabriz. The Skarn-type metasomatic alteration is the result of Qaradagh batholith intrusion into the Upper Cretaceous impure carbonates. The studied area belongs to the Central Iranian structural zone. In regional scale, the studied area is a part of the Zangezour mineralization zone in the Lesser Caucasus. Several studies (Karimzadeh Somarin and Moayed, 2002; Calagari and Hosseinzadeh, 2005; Mokhtari, 2008; Baghban Asgharinezhad, 2012; Mokhtari, 2012) including master’s theses and research programs have been done on some skarns in the Azarbaijan area considering their petrologic and mineralization aspects. However, before this study, the Avan skarn aureole has not been studied in detail. In this paper, various geological aspects of the Avan skarn including mineralogy, bi-metasomatic alteration, metasomatism and mineralization during the progressive and retrograde stages of the skarnification processes have been studied in detail. Research Method This research consists of field and laboratory studies. Field studies include preparation of the geological map, identifying the relationship between the intrusion and the skarn aureole, identifying the relationship between different parts of the skarn zone and also collecting samples for laboratory studies. Laboratory studies include petrography, mineralography and microprobe studies. Cameca SX100 Microprobe belonging to Geological Survey of the Czech Republic was used in order to determine the chemical composition of the calc-silicate minerals such as pyroxene and garnet in garnet skarn and pyroxene- garnet skarn sub-zones. Discussion and conclusion Qaradagh batholith is composed of discrete acid to mafic phases including gabbro, diorite, quartz diorite, quartz monzonite, quartz monzodiorite, tonalite, granodiorite, monzogranite and granite porphyry which is dominated by granodiorite-quartz monzonite. Granitoids of this batholith are metaluminus, high K calc-alkaline I-type granite (Mokhtari, 2008). The Avan Cu-Fe skarn is related to the intrusion of granodioritic-quartz monzonitic part of the Qaradagh batholith into the Upper Cretaceous flysch- type rocks consisting of biomicrite, clay limestone, marl, siltstone and mudstone. The Avan skarn consists of three zones of endoskarn, exoskarn and marble. The main Cu-Fe mineralized zone is related to the exoskarn zone, which has 600 meters of length and 50 meters of thickness, respectively. The Exoskarn zone consists of garnet skarn, pyroxene-garnet skarn and ore skarn sub-zones. Garnet, belonging to ugrandite series (Ad53-89) with more than 50 percentage in volume, is the most important anhydrous calc-silicate mineral in the garnet skarn and the pyroxene-garnet skarn sub-zones. Some of the garnet crystals are zoned and their chemical composition changes toward the rim to almost pure andradite (Ad99). Clinopyroxene which has diopsidic composition (Di75-96), is another anhydrous calc-silicate mineral in the exoskarn zone with an abundance that reaches up to 50 percent in volume in pyroxene-garnet skarn sub-zone. The ore skarn sub-zone is located toward the outer part of the exoskarn zone and close to the border of the marble zone. The abundance of ore minerals in this sub-zone reaches up to 50 percentage in volume and includes magnetite, hematite, pyrite, chalcopyrite, bornite, malachite and goethite among which pyrite is the most abundant. In this sub-zone, anhydrous calc-silicate minerals of garnet and clinopyroxene have undergone intensive alteration and are replaced with hydrous calc-silicate (epidote and tremolite- actinolite), oxide (magnetite and hematite) and sulfide (pyrite, chalcopyrite and bornite) minerals. Based on the textural and mineralogical studies, the skarnification processes in the studied area can be categorized into two main stages: 1) prograde and 2) retrograde. During the prograde stage, the heat flow of the granitoid has caused isochemical metamorphism and changing more pure limestones to marble and marlly limestones to skarnoid (metamorphism and bi-metasomatism). The high temperature magmatic fluids have caused prograde metamorphism during which anhydrous calc-silicate minerals including garnet and pyroxene have appeared. During the early retrograde stage, i.e. the mineralization sub-stage, lower temperature hydrothermal fluids have caused hydrolysis and carbonization because of which anhydrous calc-silicate minerals along with their fractures and microfractures are changed to hydrous calc-silicate (epidote and tremolite-actinolite), oxide (magnetite and hematite), sulfide (pyrite, chalcopyrite and bornite) and carbonate (calcite) minerals. During the late retrograde stage, relatively low temperature fluids have altered anhydrous and hydrous calc-silicate mineral assemblage formed during the previous stages into a very fine grained mineral assemblage including clay minerals, chlorite and iron hydroxides. Presence of replacement textures in ore minerals and anhydrous calc-silicate minerals accompanied with open filling textures in the anhydrous calc-silicate minerals, for example oxide and sulphide veinlets within the garnet crystals, indicate that the mentioned ore minerals have been simultaneously generated with hydrous calc-silicate minerals (epidote and tremolite-actinolite) during the early prograde stage. The presence of minor amounts of wollastonite among the mineral assemblage of the Avan skarn, intergrowth of garnet and pyroxene, absence of reaction rim between garnet and clinopyroxene and absence of replacement textures indicate that these minerals have been simultaneously generated within the temperature ranges of 430–600 ºC and ƒO2 > 10-26, respectively. Acknowledgements The authors are grateful to the Journal of Economic Geology reviewers and editors for their constructive suggestions to the manuscript. Reference Baghban Asgharinezhad, S., 2012. Investigation of genesis, mineralogy and geochemistry of Fe-Cu skarn in Astamal area, NE Kharvana, Eastern Azarbaijan. MSc. Thesis, University of Tabriz, Tabriz, Iran, 185 pp. (in Persian with English abstract) Calagari, A.A. and Hosseinzadeh, G., 2005. The mineralogy of copper-bearing skarn to the east of the Sungun-Chay River, East-Azarbaijan, Iran. Journal of Asian Earth Sciences, 28(4-6): 423-438. Karimzadeh Somarin, A. and Moayed, M., 2002. Granite and gabbro-diorite associated skarn deposits of NW Iran. Ore geology reviews, 20(3-4): 127-138. Mokhtari, M.A.A., 2008. Petrology, geochemistry and petrogenesis of Qaradagh batholith (east of Syahrood, Eastern Azarbaijan) and related skarn with considering mineralization. Ph.D. Thesis, Tarbiat Modares University, Tehran, Iran, 347 pp. (in Persian with English abstract) Mokhtari, M.A.A., 2012. The mineralogy and petrology of the Pahnavar Fe skarn, in the Eastern Azarbaijan, NW Iran. Central European Journal of Geosciences, 4(4): 578-591.

The precambrian gneisses are widely distributed in the Jangsu area. This study focuses on the metamorphic mineral assemblages and metamorphic P-T conditions of the gneiss. We have analyzed garnet, biotite and plagioclase among the gneiss through the EPMA analysis, and calculated the metamorphic temperature and pressure accordingly. The metamorphic temperature was estimated by the average of values from the garnet and biotite formulas, and the metamorphic pressure by value of the Hoisch(1990) geopressured on garnet-biotite-plagioclase. The mineral sample we examined shows garnet-biotite-plagioclase-quartz composite and garnet-plagioclase-orthoclase-quartz composite. Garnet shows almandine-pyrope solid solution in general, while porphyroblastic gneiss shows almandine-grossluar solid solution. The fact that the abundances, observed by garnet profile, are almost identical in both the central region and the outer egion indicates that the crystal was developed uniformly. There is almost negligible variance in biotite on metamorphic grade, and andesine is observed in plagioclase. The metamorphic temperature and pressure from EPMA analysis and its indications are as follows: the middle-temperature, high-pressure metamorphism (, 6.9-10 kbar) ensued in the beginning, and then was followed by the high-temperature, middle-pressure(, 2.7-5.9 kbar) to (, 3.1 kbar) retrograde metamorphism.

O Gnaisse alcalino de Serra do Matola - São Jõao Del Rei

Retrograde breakdown products of cordierite are commonly termed pinite, but their modes, compositions and formation conditions are only poorly known. A systematic study on pinitised cordierite from high-temperature metamorphic pelites of the Schwarzwald and the Bayerische Wald using electron microprobe (EM), transmission electron microscope (TEM), scanning electron microscope (SEM), x-ray diffraction, petrographic microscope and Fourier Transformation Infrared Spectroscopy (FTIR) investigations was carried out. On the basis of composition, phase assemblage, textural position and grain size four pinite types (border, mat, fissure, isotropic type) were distinguished that formed by distinct allochemical processes under different pressure-temperature conditions at different times. They probably represent general features of cordierite breakdown. Border-type (b-type) pinite consists of muscovite and biotite, formed at 350 - 550 °C from a K+-bearing fluid, most likely derived from the breakdown of K-feldspar to muscovite and quartz. B-type pinitisation may be related to granite intrusion in the Carboniferous. Mat-type (m-type) pinite encompasses chlorite-muscovite pinite and complex m-type pinite, bearing clay minerals, and is in general considered to represent the alteration of cordierite by an alkli-bearing fluid. Petrogenetic grids define an upper T limit for chlorite-muscovite of around 500 - 550 °C. Complex clay m-type pinite of this study has the principal constituents chlorite, berthierine, I/S R1 and I/S R0, Na/K-illite and random chlorite/berthierine mixed-layers. The observed features are similar to those of non-equilibrium assemblages from the early and the late diagenetic zones as complex m-type pinite lacks stable equilibrium and should have been formed much below 200 °C. In this case, its genesis is a complex two-stage stage process, with chlorite and I/S R1 representing the primary product of m-type pinitisation. Secondary berthierine and I/S R0 are formed by the replacement of chlorite and I/S R1 driven by the infiltration of an additional very low-grade fluid. M-type pinitisation could be related to low-temperature meteoric alteration of granites. F-type pinites form alteration veins penetrating intact cordierite. They are filled with tiny (< 20 - 250 Å in thickness), randomly oriented and randomly related, highly imperfect crystals, floating in an amorphous matrix. The principal crystals or packets of layers are berthierine, illite and/or I/S R1, smectite and chlorite, mixed-layers including 7/14 Å, 10/14 Å, I/S and complex mixed-layering of 7 Å, 10 Å and 14Å layers. Randomness in orientation of the crystallites points to in situ crystallisation of packets of layers directly either from a solution present in alteration veins or from a pre-existing gel-like material as more stable secondary phases. The occurrence of corresponding optical and chemical zonation pattern let the author assume that corresponding processes of leaching and repolymerisation acting in discrete band-like zones are the principle mechanism of f-type pinitisation. Lack of perfect and homogeneous structures and enhanced thickness of individual packets of layers indicate f-type pinitisation to result from cordierite alteration at very low-grade conditions. I-type pinite is fairly homogenous in BSE images and isotropic under crossed polars. Compared to cordierite, i-type pinite is depleted in most elements but networkforming species (Si and Al) and potential interlayer cations with K and Ca being most abundant (Ca > K). A dramatic fall in Si counting rates occurred during electron beam exposure at standard conditions, suggesting that i-type pinites may represent amorphous gel-like highly hydrated material. This assumption is confirmed by the occurrence of fissures penetrating i-type pinite, which are strongly suspected to represent dehydration shrinkage fissures. Unfortunately, TEM could not give further evidence. Therefore, the phase inventory and structural state of i-type pinite remain unclear. I-type pinitisation is probably a leaching process at very low temperatures (weathering) with network-modifying species being preferentially dissolved. H2O and CO2 contents of partly pinitised cordierite were determined by in situ FTIR measurements in order to detect late-stage modifications of the cordierite channel compositions, possibly related to pinitisation. The volatile contents of all investigated samples were found to be consistent with equilibration with a H2O undersaturated melt, during high-grade metamorphism. Therefore, a systematic change of cordierite channel volatile contents as a key step within the process of pinitisation is not indicated. Nevertheless, the crystallographic control of f-type pinitisation implies that the channel structure determines the c axis to be the preferred direction of cordierite dissolution/leaching.

Origin of S-type granites coeval with I-type granites in the Hellenic subduction system, Miocene of Naxos, Greece

This thesis investigates metamorphic controls on the distribution of metals and metal-transporting ligands in the mid to lower continental crust. The metal content of the mid crust is investigated by evaluating the mobility of elements during chlorite devolatilisation in the upper greenschist facies. This interval of metamorphic fluid production was found to coincide with the pyrite-pyrrhotite transition; a reaction that liberates sulfur into the metamorphic fluid and is thought to be important for scavenging gold to form orogenic Au deposits. The pyrite-pyrrhotite transition field was found to extend to higher temperatures (500 – 550 °C) in a natural system than previously described by thermodynamic modelling. Mass balance calculations suggest that S-bearing metamorphic fluids produced during simultaneous chlorite and pyrite breakdown partially extracted Au, As, Bi, Sb, Mn, and W from the rock. Conversely, the uptake of Cu, Pb, Zn, Ni, Co, Ga, Ge, Mo, Tl, V, and Cr by silicate and sulfide minerals acted to retain these metals in the deep crust.At greater crustal depths, biotite devolatilisation causes melting of lower-crustal rocks. Biotite is stabilised to higher temperatures by uptake of the metal-transporting ligand F, which is controlled by Mg#, and factors that affect Mg# in biotite, including bulk composition, coexisting mineralogy, and preferential removal of Fe over Mg during partial melting. Chlorine retention in biotite is inversely correlated to F, such that initial biotite dehydration produces a slightly Cl-enriched, F-poor melt, enriching residual biotite in F. At final dehydration of biotite, especially in MgO- and K2O-rich bulk compositions, F-rich biotite breaks down at lower-crustal conditions to produce a hot, dry, F-rich and Cl-poor granitic melt. The amount of F released into such a melt from only moderately F-enriched biotite is sufficient to form highly F-enriched melt, typical of A-type granites. These metamorphogenicmelts may be important for transport of REE and some HFSE to the upper crust due to the tendency of these elements to be enriched in F-rich melts.An investigation into mobilisation/retention of base metals during ultra-high temperature metamorphism found that element mobility is primarily controlled by the tendency of elements to partition into residual minerals. Sulfide and major silicate minerals in Rogaland metamorphic aureole rocks are sources of metals for uptake into melts produced by biotite dehydration. Copper, Pb, and Zn, are hosted in major silicate minerals at high to ultra-high temperatures, and therefore these are primarily controlled by metamorphic reactions amongst the major silicate minerals. Retention of base and precious metals can occur at high to ultra-high temperatures by shielding in quartz veins, preferential removal of silicate melt, and reaction with residual minerals. These findings suggest that metapelitic rocks in the lower crust may be viable source region for ore deposits.The work presented in this thesis identifies metamorphism as a key driver of metal redistribution in the mid and lower continental crust. Our understanding of the formation of metamorphogenic and crust-derived ore deposits is improved by understanding the relationships between fluid/melt loss, residual mineral growth, and consequent metal partitioning.

Archean greenstones represent a large percentage of worlds total gold endowment and are actively mined on every continent barring Antarctica. Greenstone-hosted gold deposits often have complex deformation and alteration histories, and a general deposit model remains controversial. The aim of this thesis is to improve our understanding of gold mineralisation in Archean greenstone belts, based on a comprehensive case study of the world-class Geita Hill deposit in Tanzania. Geita Hill is one of the largest gold deposits within the Geita Greenstone Belt in north-western Tanzania and has been mined as an open pit since 2002. The deposit is hosted within a greenschist facies metamorphosed and complexly deformed sedimentary package dominated by ironstone and intruded by diorite dykes. The gold mineralisation is spatially associated with the Geita Hill Shear Zone which, is a NE-trending, moderately west dipping deformation zone of discontinuous shear fractures. Detailed structural studies have defined a deformation history for the deposit, providing an opportunity for an in-depth study of the hydrothermal alteration and fluids associated with gold mineralisation. The first component of this thesis builds a paragenetic framework for the Geita Hill deposit. The regional metamorphism is characterised by biotite + chlorite + actinolite + K-feldspar + magnetite ± pyrrhotite ± pyrite indicating upper greenschist facies metamorphism. The gold-related alteration overprints the regional metamorphism and is characterised by silicification and sulfidation fronts that end within one meter of the mineralised zone. Locally, the silicification and sulfidation of the wall rock occurs along a series of mineralised quartz veins which have a sub-vertical dip and tend E-W. Paleostress analysis of the mineralised shear fractures of the Geita Hill Shear Zone suggests vertical maximum compressive stress (σ₁) and northerly extensional stress (σ₃) consistent with the orientation of the mineralised quartz veins and indicating N-S extension. The composition of the mineralised quartz veins is characterised by quartz + biotite + K-feldspar + pyrite, which also overprints the metamorphic mineral assemblage. Gold is closely associated with secondary pyrite and occurs as free gold and gold tellurides (sylvanite, calaverite and nagyagite). It occurs mainly as inclusions in pyrite and as invisible gold in pyrite but gold inclusions in biotite and along quartz grain boundaries are also present. The gold-bearing pyrite is associated with secondary biotite and K-feldspar. Two distinct textural styles of auriferous pyrite can be distinguished: inclusion rich subhedral pyrite and inclusion free euhedral pyrite. It is common for the inclusion rich pyrite to have thick rims of inclusion free pyrite. The mineralising alteration is overprinted by barren, multi-phase quartz-carbonate, and carbonate-chlorite veins. This alteration is characterised by the assemblage calcite + siderite + chlorite ± quartz ± pyrite ± barite. The thesis then builds on the paragenetic framework through silicate and sulfide geochemistry. Biotite was identified as a primary mineral both in the metamorphic assemblage and gold-related hydrothermal alteration assemblage. The study of silicates was conducted through detailed core logging, petrography, SEM mineral identification of alteration assemblages, SWIR measurements and microprobe analyses of biotite in order to identify the nature of the mineralising fluid and its spatial effect across the Geita Hill deposit. Results show that the mineralised assemblage is slightly more oxidized (pyrite + magnetite) compared to the metamorphic background (pyrrhotite + magnetite). The intense sulfidation within the ore zone resulted in the formation of Mg-rich biotite, which grades into more Fe-rich biotite away from the ore zone. This change in biotite composition can be detected using short wavelength infrared spectra, though a shift in the Fe-OH 2250 nm absorption feature to lower wavelengths. This shift is also correlated with an increase in gold grade within the mineralised zone. Halogen chemistry of the biotite implied the presence of multiple hydrothermal fluids during mineralisation, suggesting that within the ore zone a metamorphic fluid in equilibrium with the host rock was overprinted by and mixed with an infiltrating fluid that was enriched in fluorine. Study of the sulfides in the deposit was conducted through laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis of pyrite and magnetite grains. The gold distribution correlates well with Te, Ag, Bi and Pb consistent with its occurrence as micro- and nano-inclusions of gold bearing telluride minerals. As, Co and Ni distribution in pyrite grains hosted in quartz veins is patchy, whereas in pyrite grains from ironstone and diorite these elements show zoning characteristic of growth pattern. Elements including As, Ni, Co, Cu and Zn appear to be dominantly locally derived, and remobilised into the pyrite during sulfidation. The concentrations of these elements are highly lithologically controlled, and they are not consistently incorporated into the pyrite after initial stages of growth. Au, Ag, Te, Sn, Bi and Pb appear to be dominantly externally derived, and closely correlate in all varieties of pyrite. The Se content is typical of pyrite from Archean gold deposits (~30ppm) and reflects to an average temperature of ~340°C for the mineralising fluid on the basis of temperature dependent incorporation into pyrite. Lastly, the gold-bearing hydrothermal fluids were studied directly through detailed microthermometry and raman microspectroscopy, and three principle fluids were identified: (1) A low salinity, carbonic-rich (XCO₂ > 0.8) fluid with minor N₂ (XN₂/(XCO₂ + XN₂) 20 wt.%; NaCl/(NaCl+CaCl₂) mass ratio > 0.45), aqueous brine that was interpreted to be magmatic in origin; and (3) A low salinity (NaCl < 5 wt.%) aqueous fluid that was interpreted to be meteoric in origin. Preserved fluid assemblages imply mineralisation occurred at pressures of less than 2 kbar, likely from 1.4 to 1.7 kbar, at temperatures of approximately 350 °C. C-O-H fluid modelling of the carbonicrich fluid has constrained ƒO₂ fluid to 1.5-1.8 log10 units above ƒO₂ FMQ corresponding to absolute values of 10- 30.5 bar. The gold was likely transported in the high salinity brine as Au-bisulfide complexes with tellurium, potentially introduced as a vapour. Deposition of Au was triggered via interaction of gold-bearing fluids with the relatively reduced Fe-rich host rocks and the low salinity CO₂-rich fluid.